1. Field of the Invention
This invention generally relates to a photographic camera, and, in particular, to an electrically powered film feeding type camera. More specifically, the present invention relates to a photographic camera in which the winding of film to a spool and the rewinding of the film from the spool are automatically carried out by driving to rotate an electric motor in the normal direction and in the reversed direction, respectively.
2. Description of the Prior Art
A motor-driven film feeding camera in which the winding of film to a spool and the rewinding of the film from the spool are carried out by the normal rotation and the reversed rotation of an electric motor, respectively, is known in the art.
In such a prior art motor-driven film feeding camera, individual winding and rewinding operations of film are automated; however, a manual operation is sill required to initiate the motor-driven rewinding operation after the entire length of the film has been wound. One of such a manual operation is to disconnect the sprocket and the spool from the driving system during rewinding operation and at the same time to connect the rewinding system to the driving system. In other words, when the film is to be rewound into a film cartridge, the operator has to manually disconnect the driving system from the spool in the prior art. Accordingly, the accordance with the prior art, although each of the winding and rewinding operations of the film is automated, the entire film advancing operation is not fully automated, so that there is still a lack of convenience in using cameras.
When it is desired to carry out the film rewinding operation without requiring the manual shifting in the mechanical structure, it may be so structured that the film winding operation is carried out by causing an electric motor to be driven to rotate in the normal direction and the film rewinding operation is carried out by causing the motor to be driven to rotate in the reversed direction. Under the condition, in the case where the winding operation is to be carried out with a rewind fork engaged with the shaft of a film cartridge, when the film is wound to the spool by the sprocket, no problem will be created as long as the rotating speed of the shaft of the film cartridge, which rotates to unwind the film from the shaft, and the rotating speed of the rewind fork, which is driven to rotate, are the same. However, the film stored in a film cartridge is sometimes loosely wound around the shaft of the cartridge and sometimes tightly. Moreover, the length of the film unwound from the cartridge shaft when it makes one revolution varies consistently; whereas, the length of the film advanced by the sprocket per unit time stays constant as long as the sprocket is driven to rotate at constant speed.
Under these circumstances, several disadvantages are expected to be encountered as will be described below.
(1) In the case where the rotating speed V.sub.1 of the cartridge shaft transmitted by the sprocket via the film is larger than the rotating speed V.sub.2 of the cartridge shaft transmitted by the rewind fork. In this case, the slack of the film inside of the cartridge is gradually taken up, and eventually the film is set under tension between the sprocket and the cartridge shaft whereby the film could be torn away in the worst case.
(2) In the opposite case of the above case (1). This is the case where V.sub.1 is smaller than V.sub.2. In this case, the slack of the film inside of a film cartridge is increased, so that the film comes to be pressed against the inner wall of the cartridge. In the end, the cartridge shaft starts to wind the film in the reversed direction from its trailing edge which is fixed to the cartridge shaft. If this happens, that portion of the film wound around the cartridge shaft in the reversed direction can no longer be wound to the spool.
In order to avoid the above-mentioned defects, the power transmission relation between the cartridge shaft and the rewind fork may be disconnected. For example, a one-way clutch may be provided in the rewind power transmission system including the rewind fork. However, this is effective only when the rewind power transmission system is designed to satisfy the condition that V.sub.1 remains smaller than V.sub.2 during the time period from the initiation of the film winding operation till the completion of winding of the entire film length, and, thus, it is limited in structure. On the other hand, in the case of rewinding operation, it is necessary either to disconnect the sprocket completely from the driving system or to provide a friction mechanism to absorb the veloctiy difference of V.sub.2 -V.sub.1, which could cause complication in terms of structure required or energy loss due to friction. Another approach would be to provide the rewind fork such that it may be engaged with or disengaged from the shaft of a film cartridge, whereby the rewind fork is to be engaged with the cartridge shaft only when the film is to be rewound into the cartridge. In this case, however, since the rewind power transmission system is also driven at the time of winding operation, this will present other disadvantages such as increased energy consumption.
In a motor-driven winding and rewinding camera, it is usually so structured that the loading or setting of film is automatically carried out by closing the back cover of the camera after positioning the leading portion of the film at a predetermined position. However, it happens often times that the film is not set as expected for various reasons. For example, the operator may fail to position the leading portion of the film correctly. Thus prior art cameras are disadvantageous because no measure is taken against improperly loading the film. Another disadvantage of such prior art cameras is that the motor-driven film winding and rewinding operations may be carried out only when film is loaded in the camera.